This project is exploring relationships between candidate neurotransmitters and the types of sensory fibers entering the spinal cord in a model system, the Atlantic stingray, Dasyatis sabina. The advantages of the stingray are (1) an absence of unmyelinated fibers in the spinal cord, making it a simplified system, (2) the sensory and motor fibers are separated in the spinal nerves, (3) the coarse and fine myelinated fibers project the separate areas in the spinal cord. Amino acid and peptide neurotransmitter candidates are being studied. Presence and release of these candidate substances is being studied. The receptive field properties and fields of termination of the sensory fibers in the spinal cord are being determined. Pharmacological agonists and antagonists of candidate substances are to be determined. The system may yield a satisfactory demonstration that glutamic acid acts as a neurotransmitter in the central nervous system. This study will enhance our understanding of neurotransmitters of sensory pathways in the spinal cord of higher vertebrates, including man.